Eversion is a fun little game by Zaratustra. It's a normal sidescolling platformer with a twist -- If you stand in certain spots and press the action button, you can swap between two worlds.
These "worlds" are strung out in a whole series. Except under special circumstances(i.e. level 4 jumps you from world 1 to world 5, but also levels 3 and 6 could potentially "jump" one or more worlds if you modified the source code, which is easy to do.. You can only get from world 1 from world 2, you can only get to world 2 from worlds 1 and 3, and so on, up to 8. To get into world 8, you need to have all 240 gems, which are scattered around the level.
These levels are important because each one is slightly different to the previous ones -- in world 1 you can't walk through trees but you can walk through clouds, in world 2 you can't walk through either, in world 3 the trees all die so you can walk past them but not the clouds, in world 4 you can walk through both but are able to break bricks from below, and so on.
Additionally, each world is a little darker and creepier than the previous ones.
Eversion has a number of easter eggs.
When you hit the happy block in level 4, it jumps you from world 1 to world 5, allowing you to progress, because in world 5 you can walk through branches, and you can't do that in world 1. Because the happy blocks are all sad in world 5, you can't get any gems out of them, but you CAN in world 4.
Anyway, later on in level 4 you can swap from world 5 to world 4 -- going back to the beginning shows that the branches that were destroyed in world 5 are still there in world 4 -- so you can't get through, BUT, if you modify the source code to let you fly, you CAN get back to right at the beginning. This allows you to hit the happy block (well, in world 4, more of a neutral block) to get a gem. Picking up this gem crashes the game.
Oddity in level 3Edit
If you modify the source code so you start in world 7 on level 3, you will see a gem counter halfway through. This gem counter is actually a spot where you can access world 8 -- normally unseen until level 7 because you can't go deeper than world 4 in level 3.
Now for the part you want to hear about. These secret levels exist in the source code, but can't be accessed without modifying the source code. 9, 0, and -1 are probably worlds either used for testing or worlds that were going to be in the game but were scrapped. The others are most likely just glitches.
World 9 is a world with a black background, and where everything is invisible, and the enemies look like you. Otherwise acts like world 8.
Worlds 0 and -1Edit
These are worlds where everything is a white outline of itself. Both act like world 1. world 0 is black with glitched-up graphics and world -1 is purple.
Worlds -2 through -8Edit
These are just worlds that all act like world 1. They look like worlds 7 through 1 but everything is invisible. Good luck!